The present invention relates to high speed acquisition and storage of data from transient electrical signal waveforms sampled in time at a multiplicity of points. The invention particularly relates to situations where it is desired to collect and store data in real time in a mass storage unit such as a magnetic disk unit, but where the data rate is in excess of the apparent rate at which data can be transferred to mass storage.
The subject transient recorder was developed specifically in the context of recording the output of a time-of-flight (TOF) mass spectrometer, but has other applications as well. In particular, the subject invention has application in any transient data recording situation where repetitive transients occur at a relatively high rate but the information from one transient to the next changes at a relatively slow rate. As another example, the present invention has application where a chemical system is pulsed with a laser to produce decaying fluorescence output pulses. However, for purposes of example and to facilitate description, the invention is described herein in the context of TOF mass spectrometry.
In a time-of-flight mass spectrometer, sample ions are produced and then extracted and accelerated by an accelerating voltage applied to suitable acceleration electrodes. A typical value of accelerating voltage is 3.5 kV. Constant energy and constant-momentum acceleration modes are known.
In either case, lighter (lower mass) ions are accelerated to higher velocities than the heavier ions. The ions then enter a drift region or flight tube which establishes an ion path length 1, and which is followed by an ion detector. In the drift region, the ions separate along the ion path as a function of their individual velocities and thus arrive at the ion detector at different times depending upon their velocities, and therefore, depending upon their mass.
To permit measurement of flight time, ions in a time-of-flight mass spectrometer are bunched, typically by means of a pulsed ion source, and all ions of a given bunch enter the drift region at substantially the same position and time. By correlating ion pulsing or bunching with arrival time of various ions at the ion detector, the time-of-flight of each individual ion or group of identical-mass ions can be determined. Ion velocity follows from the simple relationship: EQU (Velocity)=(Path Length)/(Time-Of-Flight).
From velocity, ion mass can be calculated, taking into account the characteristics of the ion accelerator.
With an ion path or flight length 1 of 1.0 meter, all ions from one pulse of the ion source, ranging from 1 to 1000 mass units, reach the ion detector within 40 microseconds. Many individual ions of any given mass may reach the detector at substantially the same time, ranging up to several hundreds of ions. The source pulses are repeated at a rate in the order of 10 to 25 kHz.
The output of the ion detector is a transient waveform for each source pulse. Each transient waveform has a magnitude which varies as a function of time, with peaks of the waveform along a time axis corresponding to different masses of the various sample ions.
It will be appreciated that the data rate is extremely high, much faster than can be stored by any known mass storage unit (i.e., a magnetic disk unit). For example, in each transient there may be as many as 16,000 relevant sample points in time (or "time bins"). At a source pulse rate of 10 kHz, these two factors give a data rate of 1.6.times.10.sup.8 items of information per second. Moreover, for any given time bin up to several hundred ions may be arriving which can be represented by a data word of eight binary bits. Optimally, every single ion arriving at the ion detector can be resolved with intensities of 255 ions per bin or less. This last factor increases the potential data rate to 1.3.times.10.sup.9 bits per second. Clearly this is too fast for known mass storage techniques.
Commonly-available commercial time-of-flight mass spectrometers record detected ion current intensities by sampling techniques. Ion current is sampled during only one arrival time for each source pulse. A sampling window or time slice (aperture time) is established and the delay from the extraction pulse to this window is slowly scanned over all arrival times of interest while the source is repetitively pulsed, thereby recording a complete mass spectrum of the sample under study by collecting the ion intensities for each successive arrival time. This technique is known as Time-Slice Detection (TSD).
Additionally integrating forms of time-slice detectors have been employed, known as "boxcar integrators". The boxcar integrator is triggered for each ion pulse, and integrates ion current during the same aperture time at a constant arrival time for a number of pulses. The arrival time can either be constant or be slowly scanned.
Integration itself represents a means of data flow reduction where the information contained in successive pulses is changing slowly, as is the case in many TOF mass spectrometry applications. Although the data pulses are occurring at a 10 kHz rate, the spectrum each pulse detects is changing at a much slower rate, and time-resolved data from 10 to 1000 scans can be integrated or averaged without loss of actual information.
Time-Slice Detection has the disadvantage of losing most of the information available in the ion beam since aperture time is a small fraction of the total time over which ions are arriving at the detector. This creates a potential problem where sampling times or sample quantities are limited. Accordingly, various devices for Time Array Detection (TAD) have been proposed, known variously as "transient recorders" or "digital transient recorders". Such recorders, rather than responding to a single time slice relative to the pulsed source, collect the entire output from a single source pulse in a time-of-flight mass spectrometer to produce individual time-resolved data channels for each of a multiplicity of sample points taken serially in time.
For example, Lincoln has constructed a detector system which captures a substantial fraction of the information in a single ion source pulse from a time-of-flight mass spectrometer employing a digital transient recorder having a 2K memory (Biomation Model 8100). See K. A. Lincoln, "Data Acquisition Techniques for Exploiting the Uniqueness of the Time-of-Flight Mass Spectrometer: Application to Sampling Pulsed Gas Systems", Dyn. Mass Spectrom., 6, 111-119 (1981); also published as NASA Report Tm-81224.
Prior art digital transient recorders, although offering an improvement over time-slice detection, are not capable of measuring ten thousand transients per second consistent with the ten thousand per second pulse rate typical in TOF mass spectrometry, and thus lose data as a result of spectra not collected. In particular, their data readout time is in the order of milliseconds, and is inconsistent with the 10 kHz or greater pulse rate of time-of-flight analysis. Moreover, only a limited number of time-resolved channels, for example 2000, are available in typical prior art instruments.
Just as a boxcar integrator is an integrating form of time-slice detector, integrating forms of digital transient recorders have been employed, although operating relatively slowly.
One example of such a device is known as a Computer of Average Transients, or "CAT".
As another example, the Lincoln digital transient recorder, as described in the literature cited above, has its digital output connected to a separate "Signal Averager" which functions as an integrator. For this purpose, Lincoln employs a Nicolet Model 1170 with a Model 178 plug-in unit specifically made for digital-to-digital interfacing with the Biomation Transient Recorder. As Lincoln points out, known "signal averagers" are not fast enough to acquire spectra in real time. In the Lincoln system, the rate-determining (rate-limiting) factor is the approximately three milliseconds required to dump the 2000-word memory of the transient recorder into the signal averager. This sequence of events enables only 330 transient pulses to be analyzed each second.
Up to this point, the background of the invention has been described in the context of conventional time-of-flight (TOF) mass spectrometry. Indeed, the present invention provides significant advantages in conventional TOF mass spectrometry.
There is, however, another, completely new form of time-resolved mass spectrometry with which the subject invention may be employed as an element of an overall detector system. Specifically, this new form of time-resolved mass spectrometry is disclosed and claimed in commonly-assigned U.S. patent application Ser. No. 385,114, filed June 4, 1982, concurrently herewith, by Christie George ENKE, John Timothy STULTS and John Francis HOLLAND, entitled "COMBINATION OF TIME RESOLUTION AND MASS DISPERSIVE TECHNIQUES IN MASS SPECTROMETRY". In the instruments disclosed in the above-identified application Ser. No. 385,114, time-of-flight mass spectrometry techniques are simultaneously combined with path-bending spatial dispersion in magnetic- or electric-sector mass spectrometers to improve the mass resolution or, with an ion fragmentation region, to rapidly obtain the same multidimensional mass spectral data previously obtained by tandem mass spectrometry. The technique may be identified as time-resolved magnetic or electric-sector mass spectrometry. The instrumentation generates data defining relationships between selected parent ions and daughter ions produced by fragmentation (either metastable or induced), data to differentiate stable from metastable ions, and data to improve mass resolution.
In these instruments, it is highly advantageous to rapidly and continuously collect data in real time so that the full benefits of the combined techniques can be achieved. Specifically, it is desirable to acquire and store data at a rate of 200 MHz. Moreover, for greatest sensitivity, particularly where sample quantities are limited, it is desirable that data be continuously collected and recorded, with no pauses during operation. Ideally, every single ion reaching the ion detector is recorded in its proper time-resolved channel, and no data significance is lost.
With these time-resolved magnetic- or electric-sector instruments, the information contained in successive transient output pulse changes at rates up to 1000 times per second. Thus, assuming a pulse rate of 10 kHz, information could be lost if the information in more than ten pulses is averaged, although in many cases the data of fifty or more pulses might be averaged. Further, it will be appreciated that any pause in data collection (i.e. for readout following integration) can lead to a substantial loss of significant data.